Recently, Organic Light Emitting Diode (OLED) technologies have been developed rapidly, and have become one of the most promising technologies which can replace Liquid Crystal Displays (LCDs).
FIG. 1 illustratively shows brightnesses of a conventional OLED display panel under grey scales from 0 to 40 and brightnesses of red, green and blue sub-pixels (the grey scale 255 corresponds to a brightness of 250 nits). As can be seen from FIG. 1, when the grey scale is relatively low (a low grey scale number), the brightness of the OLED display panel is quite low. For example, the brightness of the display panel when the grey scale number is 20 is only 0.94 nits, and the brightnesses of the red sub-pixel, the green sub-pixel and the blue sub-pixel are 11.47 nits, 34.59 nits, and 2.92 nits, respectively, in consideration of aperture ratios of individual pixels and the transmittance of the polarization sheet.
FIG. 2 is a schematic diagram illustratively showing a driving circuit for driving a conventional OLED display element. In the example as shown in FIG. 2, the voltage between VDD and VSS is 7.1V; when the OLED display panel displays a low brightness such as 1 nit, although this is a low brightness, some voltage across the OLED display element still exists, and one of the red sub-pixel (R), the green sub-pixel (G) and the blue sub-pixel (B) in the OLED display element which has the lowest turn-on voltage will be turned on firstly under such voltage.
FIG. 3 is a voltage-brightness graph of the red, green and blue sub-pixels. As can be seen from FIG. 3, in the conventional OLED display element, the turn-on voltage of the red, green and blue sub-pixels are not consistent. As shown in FIG. 3, the turn-on voltage of the blue sub-pixel is about 2.7V, which is higher than the turn-on voltage of the green sub-pixel, which is about 2V. The turn-on voltage of the green sub-pixel is higher than the turn-on voltage of the red sub-pixel which is about 1.7V.
Because of large red light spectrum, a difference between an energy level of a Highest Occupied Molecular Orbital (HOMO) and an energy level of a Lowest Unoccupied Molecular Orbital (LUMO) of the red light material in the light emitting layer of the red sub-pixel is relatively small, and thus the red sub-pixel is usually turned on first. Thus, the brightnesses of the red sub-pixel, the green sub-pixel and the blue sub-pixel cannot be mixed according to a ratio (for example, R:29.7%, G:60.9%, B:9.4%) by which white light can be realized. That is to say, the light obtained by mixing of the light from the red, green and the blue sub-pixels is not white light, and a color shift occurs, generally, a red shift.
In conventional technologies, the turn-on voltages of the BGR sub-pixels are different, and the turn-on voltages are relevant to the widths of the band gaps of the materials of the RGB light emitting layers. The low gray scale is the brightness under a low voltage, if the blue sub-pixel is turned on, even though the voltage mainly occurs across the blue sub-pixel, such voltage can be applied to the green and red sub-pixels via a hole layer due to the good conductivity of the hole layer. The turn-on voltage of the red sub-pixel is relatively low, and when the voltage drop across the hole layer is not big, the remaining voltage can activate emission of red light even with a part of the voltage missing. Thus, in the case of low grey scale, the brightness of the light emitted by the red sub-pixel cannot strictly meet the desired low brightness effect, and thereby the color under the low gray scale may become more red. Similarly, the turn-on voltage of the green sub-pixel is lower than the turn-on voltage of the blue sub-pixel, the color under low gray scale may become more green.